Science Buddies: "Ask an Expert"

The Eppler Code «PROFIL»
Professor Richard Eppler is a pioneer in the field of computational aerodynamics. He wrote his first codes using punched paper strips, the high speed main memory at these times was a magnetic drum, which could hold several bytes! - Nah, not mega- or kilo-, just bytes.

Eppler developed a very fast and elegant design method, based on conformal mapping, which is the heart of his computer code. Because an airfoil also has to operate outside of its design point(s), a fast integral boundary layer method and (for the analysis of given airfoils) an accurate third order panel method (parabolic velocity variation) was added. Furthermore the code offers possibilities to modify the geometry, to calculate drag polars, and various plotting options. Due to its early roots, the computer code has been developed as a batch code. Textual and graphical output is directed to files, which makes the FORTRAN 77 code easily portable and system independent. On the other hand, the input files are quite cryptic and hard to handle for beginners. The elaborate description of theory and code [14] even contains an (now outdated) version of the FORTRAN-IV program.

The strength of the code is the design part and the fast analysis part, which makes it very well suited for the design task. The results of the integral boundary layer method agree astonishingly well with experiments, if the Reynolds numbers are above 500'000. The design module can be used to design very smooth airfoils shapes, including the leading edge region, which is often difficult with other codes. On the other hand, the design method is quite abstract and difficult to handle for beginners.

The boundary layer analysis is performed using the calculated, inviscid (without friction) velocity distributions as input; there is no direct coupling between boundary layer flow and the external flow field. Transition prediction is performed by testing the boundary layer parameters against a set of empirically derived transition relations, which work quite well for attached flow in a wide range of Reynolds numbers.

In the low Reynolds number regime the results are usually not very accurate if a laminar separation bubble or larger separated flow regions occur. This is a result of the integral boundary layer method, which simply cannot model separation (this would require some sort of coupling between boundary layer analysis and the calculation of the external flow). The code has a option to perform a displacement iteration in order to take the displacement effects of the boundary layer into account, but there is no direct interaction, as, for example, in Xfoil. Recent (2007) additions to the code however, are an improved model of laminar separation bubbles and turbulent separation. The code itself is available for a fee directly from Prof. Richard Eppler in Germany or from his US distributor Dan Somers.

Eppler, R. and Somers, D.: A Computer Program for the Design and Analysis of Low-Speed Airfoils, NASA TM-80210, 1980.